The present invention relates to a method for converting a measured signal, which, at least in a first approximation, is related to a quantity of interest by the equation ##EQU2## (where y is the quantity that is of interest, x is the measured signal, and k.sub.1, k.sub.N and k.sub.Z are constants) into a signal that is a function of the quantity of interest.
The present invention also relates to a converter for the conversion of a measured signal which is at least approximately related to a quantity of interest according to the equation (a): ##EQU3## where y is the quantity of interest, x is the measured signal, and k.sub.1, k.sub.N and k.sub.Z are constants.
The present invention further relates to a measurement arrangement comprising a sensor for sensing a quantity of interest to be sensed, the sensor emitting a measuring signal.
The present invention still further relates to a Pirani measuring circuit comprising a bridge circuit with a Pirani element.
In the field of sensory technology, the tendency can be observed to shift more and more tasks into the measuring head or the measuring sensor device which previously had been carried out in an analyzing unit. Thus, for example, more and more active measuring bridges, signal amplifiers and processing circuits as well as linearizing circuits, analog-to-digital converters, etc. are shifted into the measuring head. For some time, this has also been observed in sensors that provide total pressure measurements. This results in the advantages that in a thermally coupled manner a better and more stable-adaptation of the analyzing electronic system to the actual sensor can be achieved in a small space; that coupled-in signal errors on the transmission path between the sensor and the analyzing unit are eliminated; and that measuring head output signals, feeds, etc. can be standardized in such a manner that various measuring heads are freely exchangeable and even different measuring head types can partially be exchanged with one another at standardized interfaces of an analyzing unit.
In some cases, active sensors of this type may be connected directly to the analog-to-digital converter input of analyzing computers.
Although one aspect of the present invention relates to measured signals in general which are a function, as illustrated and explained later in equation (4b), of a physical quantity that is of interest and that is detected by the sensor, the present invention relates particularly to the analysis of measuring signals which are sensed at hot-wire vacuum or "Pirani" vacuum gauges.
Normally, vacuum gauges have output signals which, in a manner that is specific with respect to the measuring principle, are a function of the pressure detected by the sensor. This means that the output signal of the measuring heads must first be converted into pressure values by a calibrating curve or a calibrating table.
The thermal conductivity of gases, which is evaluated, at low pressures (below approximately 10.sup.-2 mbar) as well as at high pressures (above approximately 10 mbar) will asymptotically approach constant values. Since in these ranges the conductivity dependency on the pressure becomes low and the dependency curve becomes flat, the measuring sensitivity will therefore be poor, particularly in these ranges. Furthermore, in these ranges, the measurement is increasingly susceptible to disturbances because of the poor signal-to-noise ratio which exists there.
If measuring is to take place in these ranges while using analog-to-digital converters, these converters must have a high resolution and precision for this purpose which, because of the quantization error, requires a high number of steps of the analog-to-digital converters.
From U.S. Pat. No. 4,983,863, it is known to form two signals which are proportional to the natural logarithm (ln) of two input signals while utilizing the base emitter voltage of two bipolar transistors and to subtract them so that a signal is obtained which corresponds to the ln of the input signal quotient. Thus, an output signal is provided which is directly proportional to the in of the input signal quotient.
Furthermore, reference is made to German Patent Document DE-A-37 42 334, British Patent Document GB-A-2 105 047, which corresponds to German Patent Document DE-A-32 30 405 (discussed in the following), as well as U.S. Pat. No. 2,030,956.
In one of its aspects, the present invention has the object of providing in a simple manner easily interpretable output signal characteristics from a measuring signal of the above-described type, particularly from the signal sensed by a hot-wire vacuum gauge arrangement. "Easily interpretable" means in this case that it should be possible to easily draw conclusions from these characteristics with respect to the quantity detected by the sensor which is of interest.
Furthermore, with respect to the quantity that is of interest, as in the case of a vacuum gauge with respect to the measured pressure, a large measuring range must be achieved, preferably over six decades and more, with a precision that is in the order of 10%.
These and other objects are achieved by the present invention which provides a method for converting a measured signal which, at least in a first approximation, is related to a quantity of interest by the equation (a): ##EQU4## where y is the quantity of interest, x is the measured signal, and k.sub.1, k.sub.N, k.sub.Z are constants, into a signal that is a function of the quantity of interest. The method comprises implementing the function (b): EQU ln y=prop. ([ln (x-k.sub.N)-ln (k.sub.Z -x)])
wherein prop. means proportional, and where the function is implemented in an approximated manner by at least two bipolar transistors that have base emitter voltages that are dependent on collector currents of the bipolar transistors for receiving an output signal according to the equation (c): y'=ln y, with y' being the output signal.
In comparison to a known projection for correcting the dependency between measured quantities (particularly of the voltage on a Pirani gauge) and the quantity that is of interest (in the known case, the pressure) by means of diode networks, the expenditures of the present invention are extremely low. Furthermore, the ripple of the characteristic curve is significantly reduced, and pressures can be detected with the desired precision over a significantly larger range.
In contrast to another known approach of obtaining in a limited pressure range of approximately 10.sup.-4 to 1 mbar, an output signal which is linearly dependent on the pressure by analog multiplication techniques, as known from H. R. Hidber, et al., Rev.Sci.Instrum. 47, Page 912 (1976), a significantly larger measuring range is achieved as a result of the logarithmic pressure dependency realized according to the present invention, while the signal range of analyzing amplifiers is given.
Also the use of analog logarithmizers, as known from M. Wutz, et al., "Theory and Practice of Vacuum Technology", F. Vieweg & Sohn, Braunschweig, 1988, Page 413, permits only the analysis in a pressure range of from 5.multidot.10.sup.-3 mbar to 10 mbar.
As mentioned above, according to the present invention, a signal that is of interest and that changes over more than six decades is to be detectable; that is, on a hot-wire vacuum gauge, a pressure range of at least 10.sup.-3 to 10.sup.3 mbar.
The aforementioned objects are also achieved by another embodiment of the present invention which provides a converter for the conversion of a measured signal which is at least approximately related to a quantity of interest according to the equation (a): ##EQU5## where y is the quantity of interest, x is the measured signal, and k.sub.1, k.sub.N and k.sub.Z are constant. The converter comprises a first converter bipolar transistor having a collector which is connected to receive a current proportional to (x-k.sub.N), and a second converter bipolar transistor having a collector which is connected to receive a current proportional to (k.sub.Z -x). The resultant base emitter voltages of the first and second converter bipolar transistors are subtracted from one another to form an output signal In y=prop.(ln(x-k.sub.N)-ln(k.sub.Z-x)).
According to an embodiment of the present invention, a sensor which emits a signal that is to be measured and is related to a quantity that is of interest, is combined with a converter. This combination allows a set of mutually coordinated measuring sensor devices to perform analyzing unit tasks.
The aforementioned objects are also achieved by an embodiment of the present invention which provides a measurement arrangement comprising a sensor for sensing a quantity of interest to be sensed, the sensor emitting a measuring signal, and a converter for the conversion of a measured signal which is at least approximately related to a quantity of interest according to the equation (a): ##EQU6## where y is the quantity of interest, x is the measured signal, and constants. The converter includes a first converter bipolar transistor having a collector which is connected to receive a current proportional to (x-k.sub.N), and a second converter bipolar transistor having a collector which is connected to receive a current proportional to (k.sub.Z -x). The resultant base emitter voltages of the first and second converter bipolar transistors are subtracted from one another to form an output signal In y=prop.(ln(x-k.sub.N)-ln(k.sub.Z -x)).
A second aspect of the present invention is based on a known Pirani measuring bridge circuit, as illustrated in Wutz, et al., "Theory and Practice of Vacuum Technology", F. Vieweg & Sohn, Braunschweig, 1988, Page 413. In this case, the Pirani element is connected into a branch of a Wheatstone Bridge. The output voltage of a measuring operational amplifier is fed, in the sense of a negative feedback, across the first bridge diagonal as the bridge operating voltage. The input of the operational amplifier constructed as a differential amplifier is disposed on the second diagonal of the Wheatstone bridge. In a branch of the Wheatstone bridge, a temperature compensation resistor is provided. A temperature compensation is provided because changes of the ambient temperature have the same effect on the Pirani element as pressure changes and therefore lead to errors of measurement. By means of the temperature compensation known from Wutz, the latter can be carried out precisely only in a limited pressure range.
Furthermore, it is known from German Patent Document DE-PS-32 30 405 to provide for the automatic temperature compensation on a Pirani measuring circuit in a bridge branch having a temperature-sensitive resistor as the temperature compensation element. This resistor is thermally coupled with another temperature sensitive resistor which, being connected in front of an input of an addition amplifier, feeds to the amplifier a signal derived from a reference voltage as a function of the temperature.
The second aspect of the present invention has the object, based on the known arrangement according to Wutz whose simplicity is to be maintained, of suggesting a temperature-compensated Pirani measuring circuit whose compensation, considering the measuring range, is even more precise than the much more complicated compensation circuit known from the above-mentioned patent document.
The objects of the invention are also achieved by an embodiment of the present invention which provides a Pirani measuring circuit comprising a bridge circuit with a Pirani element, the bridge circuit having a bridge branch with an intermediate tap, a voltage connected via resistors between a bridge point connected with the Pirani element and the intermediate tap, the bridge circuit including a temperature compensation element.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.